Two optical communications techniques that enable increased bandwidth density in communications systems are parallel optics and wavelength division multiplexing (WDM). In parallel optics, multiple optical data signals are typically transmitted along a multi-fiber ribbon, with a single optical signal being transmitted on each fiber. In WDM, multiple optical data signals are combined and transmitted along a single optical fiber, with each optical signal being carried on a different wavelength.
Parallel WDM combines the two communications techniques by transmitting multiple optical wavelengths through each fiber of the multi-fiber ribbon. Parallel optical transmitters are typically constructed from monolithic arrays of vertical cavity surface emitting lasers (VCSELs) operating at single wavelength. Because it is typically difficult to manufacture monolithic arrays of VCSELs operating at different wavelengths, a parallel WDM transmitter that operates VCSELs at several wavelengths is typically built out of multiple dies. It is typically advantageous for VCSELs of different wavelengths to be close together because the light from several different wavelength VCSELs typically must be combined into a single optical fiber using an optical multiplexer.